A polygon mirror is likely to have a shortcoming that a laser beam is not injected into or reflected at a correct position on a mirror surface thereof if stable and smooth rotation thereof is disturbed by a change in the operating temperature. Thus, several attempts have heretofore been made to implement stable and smooth rotation.
For example, a polygon mirror driving apparatus disclosed in Japanese Patent Publication No. Hei. 3-71690 shown in FIG. 3 is characterized as having such a constitution that the outer races of ball bearings 22a, 22b are fixed with the central portions of an upper case 20 and a bracket 21 and a shaft 23 is secured to the inner races of the ball bearings 22a, 22b, so that a polygon mirror 24 rotates together with a hub 25 fixed to the shaft 23.
In this constitution, the shaft 23 is comparatively thin in diameter and long, so that the shaft 23 is generally made of carbon steel or stainless steel with high hardness. On the other hand, since the upper case 20, the bracket 21 and a lower case 26 are complicatedly shaped, they are generally formed by die-casting an aluminum alloy, a zinc alloy, or the like that is highly fusible. Let us compare the coefficient of linear expansion of carbon steel or stainless steel having high hardness with that of aluminum or zinc that is highly fusible. The coefficient of linear expansion of carbon steel is 10.7.times.10.sup.-6 /.degree. C., that of stainless steel is 14.7.times.10.sup.-6 /.degree. C., whereas that of aluminum is 23.1.times.10.sup.-6 /.degree. C., and that of zinc is 30.2.times.10.sup.-6 /.degree. C. This data attests to the fact that there is a large difference in coefficient of linear expansion between the two material groups.
If there is such a large difference between the coefficient of linear expansion of a material of which the shaft 23 is made and those of materials of which the respective casings 20, 21, 26 are made, the casing formed by the upper case 20, the bracket 21, and the lower case 26 is displaced with respect to the shaft 23 in the axial and radial directions when the temperatures of the respective members change due to change in the operating temperature of the apparatus or due to a prolonged driving of the apparatus. As a result, the preload applied to the ball bearings 22a, 22b fluctuates, which in turn changes the rotating torque within the ball bearings. Such change in the rotational torque induces irregular rotation of a motor. The irregular rotation causes jitters which does not allow a laser beam to be injected into or reflected at a correct position on a mirror surface, adversely affecting image quality.
Further, if, in the conventional structure, the coefficient of linear expansion of the hub 25 carrying the polygon mirror 24 greatly differs from that of the shaft 23, clearance and strain tend to be produced at the interface between both members. Therefore, a material of which the hub 25 is made must have the same coefficient of linear expansion as that of the shaft 23. In addition, the shaft 23 that is thin in diameter and long as shown in FIG. 2 must have an appropriate hardness. Therefore, carbon steel or stainless steel is selected.
However, the polygon mirror is generally made of an aluminum alloy. If the shaft 23 and the hub 25 are made of carbon steel, stainless steel, or the like, strain is produced between the polygon mirror 24 made of aluminum and the hub 25, which also impairs image quality.